1. Field of the Invention
The present invention relates to a method of processing a substrate for semiconductor device, and more specifically, to a method of beveling the substrate.
2. Description of the Related Art
Conventionally, a substrate for semiconductor device has been beveled so that the circumferential edges of the front and back surfaces thereof is made substantially symmetrical. This beveling process is usually carried out using a diamond grindstone having a U- or V-shaped groove edge provided around the outer circumference thereof in such a manner that the diamond grindstone is brought into contact with the substrate for semiconductor device and they are rotated about axes disposed in parallel each other, and thus the periphery of the substrate for semiconductor device is formed to a wedge-shaped cross section. Since the wedge-shaping is performed by the beveling process, it is called a beveled portion, which is relatively smaller in size in the case of a discrete semiconductor substrate as compared with that of a semiconductor substrate for integrated circuits (IC) and mainly intends to prevent the substrate for semiconductor device from being chipped off while it is handled in the fabrication of IC.
This beveling is usually carried out in the fabricating processes of a substrate for semiconductor device just after a semiconductor monocrystal has been cut or after it has been further subjected to a lapping process. The beveling process is carried out at a relatively earlier stage of the fabricating process of the substrate for semiconductor device, because a grindstone having a grinding layer composed of relatively coarse fixed abrasive grains is used in the process, and thus the substrate is greatly damaged and coarse isolated grains are produced during the beveling process.
A semiconductor discrete device is fabricated by a triple diffusion method, wherein, first, a substrate for semiconductor device cut off from a monocrystal is subjected to a grinding and/or etching process, and the dopant to form a collector region is diffused into the bulk of the substrate through both the surfaces at an elevated temperature. Next, about a half of the thickness of the substrate is ground and removed (hereinafter, referred to as half-off) from only one surface thereof, the inner low concentrated impurity layer (same as the impurity species and concentration of the monocrystal) of the substrate is exposed, the substrate is ground while accurately controlling the thickness (usually, referred to as x.sub.1) of the low impurity layer, and further diffusions for base and emitter regions are performed through limited openings on the exposed surface to finally obtain the semiconductor discrete device. Since the beveling process is carried out to the substrate before it is subjected to diffusion or after it has been subjected thereto in the fabricating processes of the semiconductor discrete device, a technical problem arises in a conventional beveling method.
Prior art will be described based on the description of, for example, Japanese Examined Patent Publication No. 60-58579.
FIG. 3 shows a substrate 21 for semiconductor device having unsymmetrical beveled portions 21a and 21b respectively formed on the front and back surfaces thereof. The beveled portions 21a and 21b are formed using a rotary grindstone 22 shown in FIG. 4.
The beveled portion 21a and 21b of the substrate 21 for semiconductor device are arranged as follows. Assuming that the beveled portion 21a on the surface of the substrate from which a diffused layer has been ground and removed (hereinafter, referred to as a front surface) has a beveled width w.sub.1 and a beveled depth of d.sub.1, and the beveled portion 21b on the back surface has a beveled width w.sub.2 and a beveled depth d.sub.2, an arrangement is such that w.sub.1 w.sub.2, d.sub.1 d.sub.2, and the angle .theta..sub.1 between the inclining surface and the main surface of the beveled portion 21a of the front surface [.theta..sub.1 =arc tan (d.sub.1 /w.sub.1)] is equal to the angle .theta..sub.2 between the inclining surface and the main surface of the beveled portion 21b of the back surface [.theta..sub.2 =arc tan (d.sub.2 /w.sub.2)]. Note that, in this case, the beveled width w.sub.2 of the beveled portion 21b of the back surface must be set to a prescribed value or a value larger than it to prevent the substrate 21 for semiconductor device from being chipped off through handling.
According to this technology, a part of the beveled portions 21a and 21b is left on the front and back surfaces of the substrate 21 for semiconductor device even after the front and back surfaces of the substrate 21 for semiconductor device have been lapped to the levels A.sub.1 and A.sub.2 which are shallower than the beveled depths d.sub.1 and d.sub.2, the front surface has been removed (half-off) to the surface B which is shallower than the beveled depth d.sub.2, and then the substrate has been polished, whereby the chip-off of the substrate 21 for semiconductor device can be effectively prevented in the following process.
Nevertheless, the following problems arise in the prior art.
The beveled angle .theta..sub.1 of the front surface must be a prescribed value or a value less than it, because the beveled depth d.sub.1 of the front surface is made large so that a part of the beveled portion 21a is left after the substrate has been subjected to the half-off and polishing, and further the beveled portion 21b of the back surface has the beveled width w.sub.2 which is set to a prescribed value or a value larger than it, as described above. Under these circumstances, if the beveled angle .theta..sub.1 of the front surface and the beveled angle .theta..sub.2 of the back surface are set to the same value, as described above, the wedge-like periphery of the substrate is made considerably sharp. As a result, the circumferential edge of the substrate 1 for semiconductor device are liable to be chipped off.
Further, according to the substrate 21 for semiconductor device of the above technology, the beveled widths w.sub.1 and w.sub.2 of both the surfaces must be differently formed, and thus when a substrate 41 for semiconductor device (numeral 41 is used to discriminate this substrate from the substrate 21 for semiconductor device to which the beveled portions 21a and 21b have been formed) is processed to form the beveled portions 21a and 21b, as shown in FIG. 4, in such a manner that they are simultaneously formed by a grindstone 22 having the grinding surface which contour is predetermined to conform with the dimensional requirements of the periphery of the beveled substrate 21 for semiconductor device, the corner of the front surface having the larger beveled width is first abutted against the grinding surface 22a of the grindstone 22. Thereafter, the corner of the back surface having the smaller beveled width is next abutted against the grinding surface 22c of the grindstone 22. As a result, downward pressure from the grinding surface 22a is left not neutralized until after the corner of the front surface has gone toward the grinding surface 22a and the corner of the back surface is abutted against the grinding surface 22c. Therefore, the substrate 21 for semiconductor device is sometimes chipped off while it is in a beveling process.